Typical boiling water nuclear reactors include a reactor assembly which consists of the reactor vessel and its internal components including the core, core shroud, top guide assembly, core plate assembly, steam separator and dryer assemblies, and jet pumps. Also included in the reactor assembly are the control rods, control rod drive housings and the control rod drives.
The reactor vessel is a generally cylindrical pressure vessel (RPV) with a single full diameter removable head. The shroud is a cylindrical stainless steel structure located within the RPV and which surrounds the core, providing a barrier to separate the upward flow through the core from the downward flow in the annulus between the RPV wall and the core shroud. The conventional core shroud is welded to the bottom of the RPV and supports the weight of the top guide, core plate and shroud head along with attached steam separators.
Recent discoveries of unexpected circumferential cracks propagating through the thickness of the shrouds in relatively young operating BWR's has prompted a re-design of the core shrouds for future BWR's.
The primary cause of the observed cracking has been intergranular stress corrosion in the heat affected zones near many of the horizontal welds of the shroud and shroud supports. There have also been some cracks observed in the mid-belt regions of BWR shrouds, and these have been thought to be caused by irradiation assisted stress corrosion.
The current advanced boiling water reactor (ABWR) shroud, like the conventional BWR shroud, is permanently welded to the bottom of the vessel and is not intended to be removed or replaced. The shroud in the ABWR has various horizontal welds similar to the BWR shrouds in current operating plants, and therefore may also be at risk of similar stress corrosion cracking problems.
The conventional pump deck section is permanently welded in place between the shroud and the reactor pressure vessel, and it too is not intended to be removed or replaced. As with the shroud welds, the pump deck welds are also susceptible to stress corrosion cracking.